1. Field of the Invention
The present invention is directed to a diffractive coupler for use with a waveguide, especially for providing sufficient coupling within operational conditions.
2. Description of Related Art
There are many designs of optical couplers that couple light between an active element, e.g., a light source or a detector, and a waveguide, e.g., a fiber. Use of coupled systems in the field typically involves changing relative positioning between the transmitter/receiver and the waveguide. These changes may arise, for example, from vibrations in use, replacement or alteration of components in the system. Variations in coupling performance due to these changes are highly undesirable. They are generally referred to as “fiber wiggle.”
One solution for reducing fiber wiggle includes actively aligning a coupling refractive lens, often an aspheric lens, in the z-direction, where z is along the fiber axis, to maximize the power being coupled. Then the lens is moved out of focus to create a larger spot. This larger spot allows coupling performance to be maintained within a desired range when the active element and the fiber are moved relative to one another in any direction. However, the larger spot reduces the coupling efficiency.
Alternatively or additionally, fiber wiggle may be reduced by decreasing the magnification of the lens performing the coupling. To achieve maximum coupling, the magnification of the lens matches the ratio between the numerical aperture of the active element and the numerical aperture of the fiber. However, due to this large magnification, small variations in the position of the active element appear as larger shifts on the fiber. By reducing this magnification, the apparent shift is smaller, making the system less sensitive to fiber wiggle. Again, this solution allows coupling performance to be maintained within a desired range when the active element and the fiber are moved relative to one another, but with reduced resultant coupling efficiency.